This invention relates to an integrated electrical filter comprising at least one filter resistor, at least one filter capacitor and a control circuit for adjusting the value of the product of the resistance of the filter resistor and the capacitance of the filter capacitor. The control circuit comprises a phase detector, means for feeding a first signal to a first input of the phase detector, means for feeding a second signal to a second input of the phase detector, means for modifying the second signal in response to the output of the phase detector to cause the first and second signals to tend towards a given phase relationship, said modifying means including a control capacitor and a control resistor.
Such a filter has been disclosed in the Conference Proceedings of the International Solid State Circuit Conference 1985, at pages 286 and 287. In this circuit a phase lock loop is used in which the output of the phase detector modifies the frequency of a voltage controlled oscillator by varying the resistance of an FET by applying the voltage from the phase detector thereto. Corresponding resistors formed by FETs in the filter have the same voltage applied to them, so that their resistance is varied in the same manner as that in the voltage controlled oscillator. The disadvantage of this arrangement is that the FET resistances are non-linear with voltage changes and that it is difficult to match the characteristics of the different FETs involved.